Recently, along with the development of digital processing techniques for video signals, household VCRs have come into the market that are designed to provide higher resolution and better quality images. The specification and drawing of U.S. Pat. No. 5,113,262 issued May 12, 1992 to C. H. Strolle et alii and entitled VIDEO SIGNAL RECORDING SYSTEM ENABLING LIMITED BANDWIDTH RECORDING AND PLAYBACK are incorporated herein by reference. This system was developed to provide a higher-resolution home VCR that is at the same time compatible with the standard VHS system.
The video signal recording system that is described in U.S. patent application Ser. No. 5,113,262 digitizes composite video signal and subjects it to adaptive digital spatio-temporal filtering to separate a digitized luminance signal substantially free of chrominance information. This digitized luminance signal is supplied as input signal to a digital band-separation filter that generates a digital low-pass filter response to the digitized luminance signal and a digital high-pass filter response to the digitized luminance signal, which responses exhibit a crossover at a mid-frequency. The digital high-pass filter response is adaptively de-emphasized relative to the digital low-pass filter response and is then folded around the mid-frequency to appear as a reverse spectrum occupying the same portion of the baseband as the normal-spectrum digital low-pass filter response. The resulting reduced-bandwidth luminance signal with accompanying horizontal and vertical synchronizing signals is then used to frequency modulate a luminance carrier frequency, to generate a luma portion of the signal recorded on the video tape in a manner substantially in accordance with the standard VHS system procedure.
The playback circuitry for a video signal recording and playback system that is described in U.S. Pat. No. 5,113,262 includes a time-base corrector (TBC) that corrects during playback for the time-base error (TBE) in the reduced-bandwidth luminance signal recovered in analog form from the video tape by standard methods and subsequently digitized. A TBC is a device in which time-base error included in a video signal is eliminated by a memory which functions as a time-base buffer. The video signal with the TBE is written into the memory in accordance with a clock signal synchronized with the video signal with the TBE and is read out in accordance with a stable clock signal. The term "TBE" denotes the jitter introduced into a signal by the mechanical factors in a VCR recording and playback system, such as speed variations of the tape transport during recording and during playback, tape vibration, etc.
The time-base-corrected reduced-bandwidth luminance signal is unfolded, and the unfolded signal subjected to adaptive digital spatio-temporal filtering to separate a full-spectrum digitized luminance signal substantially free of folding artifacts. The high-frequency portion of the separated digitized luminance signal substantially free of folding artifacts is then re-emphasized.
The video signal recording system that is described in U.S. patent Ser. No. 5,113,262 follows in some respect the standard VHS system procedure for recording and playing chroma information in the color-under format. The standard VHS system procedure for playing back chroma information recorded in the color-under format includes measures as described in the next paragraph, which measures prevent time-base error (TBE) in the video signals from introducing color errors. For example, TBE occurs in the composite video signals to be recorded when they are supplied from another video recorder owing to speed variations of the tape transport used for playback, to tape vibration, etc. During recording, further TBE arises owing to speed variations of the tape transport used for recording, to tape vibration, etc. During subsequent playback, still further TBE arises owing to speed variations of the tape transport used for playback, to tape vibration, etc.
During recording, separated chrominance sidebands with a suppressed 3.58 MHz carrier are heterodyned with 4.21 MHz oscillations to generate complex amplitude-modulation sidebands of a 629 kHz suppressed carrier. Intervening color bursts are heterodyned down to 629 kHz during this down-conversion procedure for generating a color-under signal. The 4.21 MHz oscillations used in the down-conversion are a so-called "nervous carrier signal" supplied from a phase-locked oscillator (PLO). This PLO includes a voltage-controlled oscillator (VCO), the frequency and phase of which VCO are electrically controlled by an error signal and are synchronized to a multiple of the scan line frequency. To achieve such synchronization, the VCO is included in a corrective-feedback loop connection in which the VCO supplies its oscillations to a frequency-divider, the frequency-divider supplies a submultiple of the oscillations to the frequency and phase comparator, the frequency and phase comparator supplies its comparison results to a loop filter that determines the speed with which the error signal can be diminished by the loop connection, and the filter response is applied to the VCO for regulating the frequency and phase of its oscillations, thereby to complete the corrective-feedback loop that adjusts the frequency and phase of the oscillations from the VCO so as to diminish the error signal. If the filter response time constant is not appreciably longer than a scan line duration, the frequency and the phase of the 4.21 MHz oscillations vary in accordance with the TBE in the horizontal synchronization. In the down-conversion to 629 kHz the TBE in the chroma differentially combines with the TBE in the 4.21 MHz oscillations, so that the color-under carrier is essentially free of TBE. During playback, in an up-conversion procedure, stable 4.21 MHz oscillations are heterodyned with the color-under signal to regenerate color burst and chroma sidebands, the 3.58 MHz carrier of which is essentially free of TBE.
Of interest to the invention is the need to generate the clocking signal that is used to time the sampling of the composite video signal by an analog-to-digital converter when digitizing the composite video signal and that is subsequently used to clock the resulting digital samples into time-base corrector memory. As noted previously, TBE occurs in the composite video signals to be recorded when they are supplied from another video recorder owing to speed variations of the tape transport used for playback, to tape vibration, etc. To track any such TBE by varying the instants of sampling the horizontal scan lines picture-element-by-picture-element with the analog-to-digital converter, during digitization of the composite video signal supplied for recording, the clocking signals for the analog-to-digital converter are supplied from a phase-locked oscillator (PLO) locked to horizontal sync pulses separated from the composite video signal being digitized. This PLO includes a voltage-controlled oscillator (VCO), the frequency and phase of which VCO are electrically controlled by an error signal and are synchronized to a multiple of the scan line frequency. (The VCO may in fact be a current-controlled oscillator instead, as will be appreciated by those familiar with PLO design.)
The oscillations of this VCO are locked in frequency to the synchronizing pulses, just like the nervous carrier used in the procedures to convert chrominance signals extracted from composite video signals to color-under signals and to convert back to chrominance signals form color-under signals. By analogy, then, clocking signals generated from the oscillations of this VCO for timing digital video processing are referred to as "nervous cocking signals" or "nervous clock signals".
To achieve synchronization to a multiple of the scan line frequency, the VCO is included in a corrective-feedback loop connection in which the VCO supplies its oscillations to a frequency-divider, the frequency-divider supplies a submultiple of the oscillations to the frequency and phase comparator, the frequency and phase comparator supplies its comparison results to a loop filter that determines the speed with which the error signal can be diminished by the loop connection, and the filter response is applied to the VCO for regulating the frequency and phase of its oscillations, thereby to complete the corrective-feedback loop that adjusts the frequency and phase of the oscillations from the VCO so as to diminish the error signal. If the filter response time constant is not appreciably longer than a scan line duration, the frequency and the phase of the nervous clocking signal supplied from the VCO to the analog-to-digital converter vary in accordance with the TBE in the horizontal synchronization. The TBE in the horizontal synchronization is an index of the TBE in the luminance signal, so the TBE in the luminance signal is compensated for when the time-base-corrector memory is read from in accordance with a stable clock.
In accordance with the invention, the 4.21 MHz sinusoidal signals used in the down-converting and up-converting procedures associated with color-under signals are not generated directly using a phase-locked analog oscillator oscillating at a 4.21 Mhz frequency. Instead, the 4.21 Mhz frequency nervous carrier signals are derived from the higher-frequency oscillations supplied as nervous clock signals to the analog-to-digital converter used for digitizing luminance signals. This derivation can be done using a sine-wave look-up table stored in read-only memory (ROM) or by heterodyning a stable oscillator with the higher-frequency oscillations or a submultiple of those oscillations as obtained by frequency division.